Nonvolatile memories (NVM) provide flexible and low-cost data storage for many applications such as personal music devices, digital cameras, and computer hard drives. Many of these memories are both writable and erasable, allowing data to be changed post manufacture, in the field.
The writing and erasing of these memories often have different current profiles with one operation requiring substantially higher current than the other. Furthermore, one of the two operations often takes substantially longer than the other operation. As an illustrative example, the MSP430F543XA microcontroller from Texas Instruments includes a 192 kB flash memory configured into pages. According to the MSP430F543XA datasheet (SLAS655B—JANUARY 2010—REVISED OCTOBER 2010), erasing a 512B page requires up to 2 mA of current from 1.8V for 32 ms for a total required energy of about 115.2 uJ. However, writing a byte of memory may be done for much less energy: 5 mA from 1.8V for 85 us for a total required energy of 0.77 uJ.
In general, storing a new value in NVM requires both a write and an erase operation; thus, the minimum energy required to change the value is set by whichever is greater: write energy or erase energy. In applications where available energy is limited (e.g., a scavenged power supply) it is often not possible to meet both the write and erase energy requirements using existing technology.